Bound states of water in poly(vinyl alcohol) hydrogel prepared by repeated freezing and melting method

Bound state of water in poly(vinyl alcohol) (PVA) hydrogel produced by repeated freezing and melting method was investigated by thermal analysis and Raman spectroscopy. Water frozen in PVA hydrogel showed the lowering of the melting temperature depending on PVA concentration. This water can be treated as a supercooled water, of which the melting enthalpy is different from that of normal water frozen at 0 °C. Accurate melting enthalpies of supercooled water estimated by the difference of thermal capacity of supercooled water and ice allowed us to determine the weight content of supercooled bound water. The weight content of supercooled water decreased with increasing PVA concentration, and alternatively that of non-freezable water increased. The maximum content of non-freezable water was 31.4 wt% for the 50 wt% PVA gel. The pore size filled with water was successfully estimated by thermodynamical equation. The diameter is 30.2 nm for 10 wt% gel, which is comparable to that derived from other experimental methods. Raman spectroscopy was used to investigate the hydrogen bond of water. With increasing concentration, the rate of water without hydrogen bond increased. This might be interpreted by the increment of surface area of water as the pore size is reduced. This also supports that the PVA hydrogel takes the pore model filled with water.